From: Andreas Joseph Krogh <andreak@officenet.no>
To: pgsql-sql@postgresql.org
Subject: Index usage in order by with multiple columns in order-by-clause
Date: Fri, 10 Aug 2007 16:53:12 +0200
Message-ID: <200708101653.13011.andreak@officenet.no> (raw)
I have the following test-case:
CREATE TABLE test(
name varchar PRIMARY KEY,
value varchar NOT NULL,
created timestamp not null
);
create index test_lowernamevalue_idx ON test ((lower(name) || lower(value)));
create index test_lowernamevaluecreated_idx ON test ((lower(name) ||
lower(value)), created);
andreak=# EXPLAIN ANALYZE select * from test order by lower(name) ||
lower(value) ASC, created ASC;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
Index Scan using test_lowernamevaluecreated_idx on test (cost=0.00..61.58
rows=770 width=72) (actual time=0.013..0.013 rows=0 loops=1)
Total runtime: 0.127 ms
(2 rows)
andreak=# EXPLAIN ANALYZE select * from test order by lower(name) ||
lower(value) ASC, created DESC;
QUERY PLAN
--------------------------------------------------------------------------------------------------------
Sort (cost=60.39..62.32 rows=770 width=72) (actual time=0.034..0.034 rows=0
loops=1)
Sort Key: (lower((name)::text) || lower((value)::text)), created
-> Seq Scan on test (cost=0.00..23.47 rows=770 width=72) (actual
time=0.004..0.004 rows=0 loops=1)
Total runtime: 0.123 ms
(4 rows)
As the EXPLAIN-output shows, the index is not used when sort-ordering differs
in the two order-by-columns.
Is there a way I can have multiple columns in the ORDER BY clause, each with
different ASC/DESC-order and still use an index to speed up sorting?
In my application I often have a need to sort by more than 3 columns, so I'm
really wondering if there is a way to make sorting of multiple columsn (each
which may have different sort-order) use an index? Preferrably without having
to create 2^N indexes.
--
Andreas Joseph Krogh <andreak@officenet.no>
Senior Software Developer / Manager
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To: pgsql-sql@postgresql.org
Cc: andreak@officenet.no
Subject: Re: Index usage in order by with multiple columns in order-by-clause
In-Reply-To: <200708101653.13011.andreak@officenet.no>
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